Faraday rotator, optical isolator, polarizer, and diamond-like carbon thin film

ABSTRACT

New material useful in miniature, low-cost Faraday rotators, polarizers (analyzers) and magnetic substances; in Faraday rotators and optical isolators that can handle a plurality of wavelengths; and in miniaturizing, and reducing the cost and enhancing the performance of, optical isolators and various optical devices. Optical isolator ( 60   b ) as one example is configured by rectilinearly arranging a wavelength-selective Faraday rotator ( 30 ), a polarizer ( 20 ) and an analyzers ( 40 ) formed from a DLC thin film, and a magnetic substance ( 50 ) that is transparent to light. Integrally forming these using thin-film lamination technology simplifies the fabrication procedure to enable manufacturing miniature, low-cost optical isolators.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to Faraday rotators, opticalisolators, polarizers and diamond-like carbon thin films, and moreparticularly relates to—in optical communications fields—Faradayrotators for rotating light-wave polarization planes, optical isolatorsfor blocking return beams from a light source, polarizers fortransmitting only a given polarized component of light, and todiamond-like carbon thin films utilized as materials in opticalcommunications fields.

[0003] 2. Description of the Background Art

[0004] In optical communications systems constituted from optical fibersand optical elements, reflected light from optical-connector junctionsand optical circuit components is sometimes reintroduced to the lightsource. Noise produced by beams returning to a light source—especiallyto a semiconductor laser—often turns out to be a major problem indesigning optical communications systems and optical devices.

[0005] The means commonly used for blocking off the return beams is anoptical whose constituent elements are a Faraday rotator, a polarizer,an analyzer, and a magnetic part.

[0006] By virtue of the magnetic part applying a magnetic field to amagneto-optical (magneto-optical material), Faraday rotators rotate thepolarization plane of an incident light beam traveling in the directionof the magnetic field. Meanwhile, polarizers (analyzers) allow only agiven polarized light component to pass, and block components apart fromthat which is polarized.

[0007] As illustrated in FIG. 14, an optical isolator 6 is configured asan assembly of a polarizer 2, a Faraday rotator 3, an analyzer 4, and amagnetic part 5, and the non-repelling characteristics of themagneto-optical material are exploited to block the incident light frombeing reintroduced from the opposite direction. A generaloptical-isolator assembly will be more specifically described in thefollowing, while reference is made to FIG. 14.

[0008] Incident light from a light source 1 initially is filteredthrough the polarizer 2 into a polarization plane, and then transits theFaraday rotator 3, whereby the polarization plane is rotated 45 degrees.With its polarization plane rotated by 45 degrees the incident lightpasses through and radiates as it is from the analyzer 4, and in partonce more enters the analyzer 4 as a return beam and is reintroducedinto the Faraday rotator 3. The polarization plane of the return beam isagain rotated 45 degrees by the Faraday rotator 3, and with itspolarization plane having been rotated 90 degrees in total, the returnbeam is unable to pass through the polarizer 2, where the return beam isthus blocked off.

[0009] It will be understood that the arrows drawn at certain angleswith respect to the arrows indicating either the light emitted from thelight source 1 or the return beam are schematic representations of thepolarization directions of either the emitted light or the return beam.

[0010] Yttrium iron garnet (YIG hereinafter) crystals orbismuth-substituted garnet crystals have usually been used forconventional Faraday rotators (magneto-optical bodies). Furthermore, forconventional polarizers (analyzers), rutile (titanium oxide)monocrystals or glass superficially onto which silver particles areorientated in a single direction are usually used, while for themagnetic part that applies a magnetic field to the magneto-optical body,samarium-based rare-earth magnetic substances are

[0011] The YIG crystals or bismuth-substituted garnet crystals chieflyused for conventional Faraday rotators must have a certain thickness toobtain a needed Faraday rotation angle, which results in a largeexternal form. Likewise, the external form becomes large in the case ofthe rutile monocrystals and the glass onto which silver particles aresuperficially orientated in a single direction, that have been chieflyused for conventional polarizers (analyzers), and the samarium-basedrare-earth magnetic substances chiefly used as the magnetic part forapplying a magnetic field to the magneto-optical body, since they mustoccupy a certain volume. What is more, with conventional isolatorsespecially—whose basic constituent elements are a Faraday rotator, apolarizer (analyzer) and a magnetic part—has been the problem of beinglarge-sized overall.

[0012] Meanwhile, Faraday rotators, polarizers (analyzers) and magneticbodies are expensive, making conventional optical isolators in whichthese are the constituent elements cost all the more. A further problemhas been that because the individual constituent elements inconventional optical isolators are independent, their assembly processis complex, adding that much more to the cost.

[0013] Moreover, because as a general rule what determines aFaraday-rotator angle is thickness, conventional Faraday rotators canonly correspond to a single wavelength. The consequent problem too withconventional optical isolators having a conventional Faraday rotator asa constituent element has been that they basically can handle only asingle wavelength.

SUMMARY OF INVENTION

[0014] Given the foregoing, objects of this invention are: first, tominiaturize and hold down the cost of, respectively, Faraday rotators,polarizers, analyzers, magnetic bodies, and optical isolators in whichthese are the constituent elements; second, to enable a Faraday rotatorand an optical isolator to handle a plurality of wavelengths; and third,a new material useful in miniaturizing and in lowering the cost andenhancing the performance of polarizers to begin with, and of variousoptical devices.

[0015] The invention in being a Faraday rotator having wavelengthselectivity, for selectively rotating only the polarization plane ofincident light of given wavelengths, is characterized in being furnishedwith: a magneto-optical part that rotates the polarization plane ofincident light traveling in the direction of its magnetic field; and adielectric multi-layer film in which a low refractive-index layer and ahigh refractive-index layer are laminated in alternation, for localizingwithin the magneto-optical part incident light of at least onewavelength.

[0016] Preferably, the dielectric multi-layer film is characterized inlocalizing within the magneto-optical part incident light beams ofplural wavelengths.

[0017] Further preferably, the magneto-optical part is characterized inbeing constituted from a gadolinium iron garnet thin film.

[0018] Further preferably, the dielectric multi-layer film ischaracterized in being composed by laminating in alternation siliconoxide as a low refractive-index layer, and titanium oxide as a highrefractive index layer.

[0019] Further preferably, the magneto-optical part and the dielectricmulti-layer film are characterized in being formed integrally by avapor-phase process.

[0020] Under a separate aspect the invention in being an opticalisolator having wavelength selectivity, for selectively blocking onlyreturn beams from incident light of given wavelengths, is characterizedin being furnished with: a magneto-optical part for rotating thepolarization plane of incident light traveling in the direction of itsmagnetic field; a magnetic part for applying a magnetic field to themagneto-optical part; a dielectric multi-layer film in which a lowrefractive-index layer and a high refractive-index layer are laminatedin alternation, for localizing within the magneto-optical part incidentlight of at least one wavelength; a polarizer for picking out polarizedcomponents from incident beams; and an analyzer utilized in combinationwith the polarizer.

[0021] Preferably, the dielectric multi-layer film is characterized inlocalizing within the magneto-optical part incident light beams ofplural wavelengths.

[0022] Further preferably, the magneto-optical part is characterized inbeing constituted from a gadolinium iron garnet thin film.

[0023] Further preferably, the magnetic part is characterized in beingconstituted from a gallium-nitride magnetic semiconductor thin film thatexhibits ferromagnetism at room temperature and is transparent to light.

[0024] Further preferably, the dielectric multi-layer film ischaracterized in being composed by laminating in alternation siliconoxide as a low refractive-index layer, and titanium oxide as a highrefractive index layer.

[0025] Further preferably, the polarizer and the analyzer arecharacterized in being lent a structure having distributed refractiveindices, by irradiating with either a particle beam or an energy beam adiamond-like carbon thin film along a bias with respect to the film'sthickness direction.

[0026] Further preferably, the particle beam is characterized in beingan ion beam, an electron beam, a proton beam, α-rays, or a neutron beam;and the energy beam in being light rays, X-rays or γ-rays.

[0027] Further preferably, the magneto-optical part, the magnetic part,the dielectric multi-layer film, the polarizer, and the analyzer arecharacterized in being formed integrally by a vapor-phase process.

[0028] Further preferably, the polarizer and the analyzer arecharacterized in utilizing a diamond-like carbon thin film that istransparent in the light region, and that has an extinction coefficientthat is 3×10⁻⁴ or less at optical-communications wavelengths of from1200 nm to 1700 nm.

[0029] Under another aspect the invention in being a polarizer ischaracterized in being lent a structure having distributed refractiveindices, by irradiating with either a particle beam or an energy beam adiamond-like carbon thin film along a bias with respect to the film'sthickness direction.

[0030] Preferably, the particle beam is characterized in being an ionbeam, an electron beam, a proton beam, α-rays, or a neutron beam; andthe energy beam in being light rays, X-rays or γ-rays.

[0031] Further preferably, the polarizer is characterized in utilizing adiamond-like carbon thin film that is transparent in the light region,and that has an extinction coefficient that is 3×10⁻⁴ or less atoptical-communications wavelengths of from 1200 nm to 1700 nm.

[0032] According to another aspect of the invention, the diamond-likecarbon thin film is characterized in being transparent in the lightregion, and in having an extinction coefficient that is 3×10⁻⁴ or lessat optical-communications wavelengths of from 1200 nm to 1700 nm.

[0033] Further preferably, optics components are characterized inutilizing a diamond-like carbon thin film that is transparent in thelight region, and whose extinction coefficient is 3×10⁻⁴ or less atoptical-communications wavelengths of from 1200 nm to 1700 nm.

[0034] Accordingly, under this invention, miniaturizing and moreoverholding down the costs of Faraday rotators, polarizers, analyzers,magnetic parts, and optical isolators having these as their constituentelements, is made possible. Likewise, manufacturing Faraday rotators andoptical isolators that can handle plural wavelengths is made possible.Furthermore, a new material useful in miniaturizing and in lowering thecost and enhancing the performance of polarizers to begin with, and ofvarious sorts of optical devices, can be provided.

[0035] From the following detailed description in conjunction with theaccompanying drawings, the foregoing and other objects, features,aspects and advantages of the present invention will become readilyapparent to those skilled in the art.

BRIEF DESCRIPTION OF DRAWINGS

[0036]FIG. 1 is a view schematically illustrating a Faraday rotator ofEmbodiment 1 according to the invention;

[0037]FIG. 2 is a diagram representing, according to a simulation, thefunction of a Faraday rotator for a single wavelength;

[0038]FIG. 3 is a diagram representing, according to a simulation, thefunction of a Faraday rotator for two wavelengths;

[0039]FIG. 4 is a diagram representing, according to a simulation, thefunction of a Faraday rotator for two wavelengths;

[0040]FIG. 5 is a diagram representing, according to a simulation, thefunction of a Faraday rotator for two wavelengths;

[0041]FIG. 6 is a diagram representing, according to a simulation, thefunction of a Faraday rotator for two wavelengths;

[0042]FIG. 7 is a diagram representing, according to a simulation, thefunction of a Faraday rotator for three wavelengths;

[0043]FIG. 8 is a view schematically illustrating an optical isolator ofEmbodiment 2 according to the invention;

[0044]FIG. 9 is a view schematically illustrating an optical isolator ofEmbodiment 2 according to the invention;

[0045]FIG. 10 is a view schematically illustrating a polarizermanufacturing process according to Embodiment 3 of the invention;

[0046]FIG. 11 is a graph representing, according to a simulation, thefunction of a polarizer utilizing a DLC thin film;

[0047]FIG. 12 is a chart diagramming measurement results on the spectraltransmission characteristics of a DLC thin film actually fabricatedusing the parallel-plate plasma CVD method;

[0048]FIG. 13 is a chart diagramming the DLC thin film's opticalcharacteristics, calculated based on the measurement results in FIG. 12;and

[0049]FIG. 14 is a view schematically illustrating the makeup of ageneral optical isolator.

DETAILED DESCRIPTION

[0050] Embodiments of the invention will be explained in detail in thefollowing with reference to the drawings. It will be understood thatidentical or equivalent parts in the figures are labeled with the samereference marks and explanation thereof will not be repeated.

[0051] Embodiment 1

[0052]FIG. 1 is a view schematically illustrating a Faraday rotator ofEmbodiment 1 according to the invention.

[0053] This Faraday rotator 30 is furnished with, as shown in FIG. 1, amagneto-optical part 30-1 for rotating the polarization plane ofincident light traveling in the direction of its magnetic field, anddielectric multi-layer films 30-2 for localizing within themagneto-optical part 30-1 incident light of at least one wavelength.

[0054] The magneto-optical part 30-1 is constituted from a gadoliniumiron garnet (GIG hereinafter) thin film, and the dielectric multi-layerfilms 30-2 are composed by laminating in alternation silicon oxide as alow refractive-index layer, and titanium oxide as a high refractiveindex layer.

[0055] As shown in FIG. 1, the Faraday rotator 30 is constituted byarranging the dielectric multi-layer films 30-2 on either side of themagneto-optical part 30-1 to create a resonant structure. The resonantstructure of the dielectric multi-layer films 30-2 enables localizing inthe magneto-optical part 30-1 incident light of a given wavelength. Thisas a result makes it possible to selectively rotate the polarizationplane of incident light of a given wavelength.

[0056] Moreover, either adjusting the thickness of the magneto-opticalpart 30-1, or interlaminating additional dielectric layer(s) into themagneto-optical part 30-1, makes possible selectively rotating thepolarization plane of incident light of not only a single but also aplurality of wavelengths. Furthermore, adjusting the thickness andlayout of the magneto-optical part 30-1 (including such additionaldielectric layers as have been interlaminated therein) and thedielectric multi-layer films 30-2 enables controlling the wavelength,and controlling the number of wavelengths, of the incident-light whosepolarization plane is rotated.

[0057] In the following, the fact that the wavelength of, and the numberof wavelengths of, the incident-light whose polarization plane isrotated are controllable by adjusting the thickness and layout of themagneto-optical part 30-1 (including such additional dielectric layersas have been interlaminated therein) and the dielectric multi-layerfilms 30-2 will be explained using simulation results in FIGS. 2 through7.

[0058]FIGS. 2 through 7 are diagrams representing, according tosimulations, the function of Faraday rotators that selectively rotatethe polarization plane of incident light of given wavelength(s). Datafor tantalum oxide (Ta₂O₅) as a substitute for a GIG thin film, andfurther, data for silicon oxide (SiO₂) as a low refractive-index layerand for titanium oxide as a high refractive-index layer in thedielectric multi-layer film, are respectively used for the simulationsillustrated by FIGS. 2 through 7.

[0059] Transmission characteristics yielded in shining infrared light of1000 to 2000 nm in wavelength on a multi-layer film made up of thetantalum oxide, silicon oxide, and titanium oxide were calculated fromthe simulations.

[0060]FIG. 2 is a diagram representing, according to a simulation, thefunction of a Faraday rotator that selectively rotates only thepolarization plane of incident light of a single wavelength.

[0061] The multi-layer film structure for FIG. 2 may be represented as1L (1H 1L)⁵2M (1L 1H)⁵1L, wherein L represents silicon dioxide; H,titanium dioxide; and M, tantalum oxide as a GIG thin-film substitute.The coefficients attached in front of L, H and M represent the opticalfilm thickness set out by a 1500-nm wavelength design, and in practicethe physical film thickness d is expressed by

d=(¼n)λ

[0062] when the optical membrane thickness is 1L given that therefractive index of silicon dioxide is n. Further, (1L 1H)⁵ signifiesfive laminae each, ten total laminae, the titanium dioxide and silicondioxide layers being laminated in alternation.

[0063] When this multi-layer film structure is illuminated with infraredlight 1000 to 2000 nm in wavelength, as shown in FIG. 2, only incidentlight approximately 1500 nm in wavelength resonates within themagneto-optical part; and incident light in the vicinity thereof, in awavelength region of from roughly 1250 nm to 1850 nm, is blocked. That aFaraday rotator made up of the multi-layer film structure in FIG. 2 actsto selectively rotate only the polarization plane of incident light of asingle wavelength, by localizing within the magneto-optical partincident light of a single wavelength, can be ascertained from theresults of this simulation.

[0064]FIG. 3 is a diagram representing, according to a simulation, thefunction of a Faraday rotator that selectively rotates only thepolarization planes of incident light of two wavelengths.

[0065] The multi-layer film structure for FIG. 3 may be represented as1L(1H 1L)⁶5.2M(1L 1H)⁶1L. The significance of the symbols that representthe multi-layer film structure is likewise as with FIG. 2.

[0066] When this multi-layer film structure is illuminated with infraredlight 1000 to 2000 nm in wavelength, as shown in FIG. 3, only incidentlight approximately 1380 nm in wavelength and approximately 1710 nm inwavelength resonates within the magneto-optical part; and incident lightin the vicinity thereof, in a wavelength region of from roughly 1250 nmto 1850 nm, is blocked. From these simulation results, it is evidentthat plural incident beams of two wavelengths can be localized within amagneto-optical part by altering the thickness of the magneto-opticalpart in the multi-layer film structure for FIG. 2. That a Faradayrotator made up of the multi-layer film structure in FIG. 3 acts toselectively rotate only the polarization planes of incident light of twowavelengths can be ascertained from these results.

[0067]FIG. 4, like FIG. 3, is a diagram representing, according to asimulation, the function of a Faraday rotator that selectively rotatesonly the polarization planes of incident light of two wavelengths.

[0068] The multi-layer film structure for FIG. 4 may be represented as1L(1H 1L)⁶2.2M1L 2M(1L 1H)⁶1L. The significance of the symbols thatrepresent the multi-layer structure is likewise as with FIG. 2.

[0069] When this multi-layer film structure is illuminated with infraredlight 1000 to 2000 nm in wavelength, as shown in FIG. 4, only incidentlight approximately 1410 nm in wavelength and approximately 1670 nm inwavelength resonates within the magneto-optical part; and incident lightin the vicinity thereof, in a wavelength region of from roughly 1250 nmto 1850 nm, is blocked. From these simulation results, it is evidentthat plural incident beams of two wavelengths can also be localizedwithin a magneto-optical part by interlaminating dielectric layers intothe magneto-optical part in the multi-layer film structure for FIG. 2.That by interlaminating dielectric layers into its magneto-optical part,a Faraday rotator made up of the multi-layer film structure in FIG. 4acts to selectively rotate only the polarization planes of incidentlight of two wavelengths, as with FIG. 3, can be ascertained from theseresults.

[0070]FIG. 5, like FIG. 3, is a diagram representing, according to asimulation, the function of a Faraday rotator that selectively rotatesonly the polarization planes of incident light of two wavelengths.

[0071] The multi-layer film structure for FIG. 5 may be represented as1L (1H 1L)⁶2.3M1L 2M(1L 1H)⁶1L. The significance of the symbols thatrepresent the multi-layer structure is likewise as with FIG. 2.

[0072] When this multi-layer film structure is illuminated with infraredlight 1000 to 2000 nm in wavelength, as shown in FIG. 5, only incidentlight approximately 1420 nm in wavelength and approximately 1690 nm inwavelength resonates within the magneto-optical part; and incident lightin the vicinity thereof, in a wavelength region of from roughly 1250 nmto 1850 nm, is blocked. From these simulation results, it is evidentthat the resonant peak values of two wavelengths of incident light thatis localized within a magneto-optical part can be varied by adjustingthe thickness of the magneto-optical part in the multi-layer filmstructure for FIG. 4. That by adjusting the thickness of itsmagneto-optical part, a Faraday rotator made up of the multi-layer filmstructure in FIG. 5 acts to selectively rotate only the polarizationplanes of incident light of two wavelengths that are different fromthose in FIG. 4 can be ascertained from these results.

[0073]FIG. 6, like FIG. 3, is a diagram representing, according to asimulation, the function of a Faraday rotator that selectively rotatesonly the polarization planes of incident light of two wavelengths.

[0074] The multi-layer film structure for FIG. 6 may be represented as1L(1H 1L)⁶2.2M1L 1H 1L 2M(1L 1H)⁶1L. The significance of the symbolsthat represent the multi-layer film structure is likewise as with FIG.2.

[0075] When this multi-layer film structure is illuminated with infraredlight 1000 to 2000 nm in wavelength, as shown in FIG. 6, only incidentlight approximately 1450 nm in wavelength and approximately 1620 nm inwavelength resonates within the magneto-optical part; and incident lightin the vicinity thereof, in a wavelength region of from roughly 1250 nmto 1850 nm, is blocked. From these simulation results, it is evidentthat the resonant peak values of two wavelengths of incident light thatis localized within a magneto-optical part can be varied also byadjusting the thickness of dielectric layers that are interlaminatedinto the magneto-optical part in the multi-layer film structure for FIG.4. That by adjusting the thickness of dielectric layers that areinterlaminated into its magneto-optical part, a Faraday rotator made upof the multi-layer film structure in FIG. 6 acts to selectively rotateonly the polarization planes of incident light of two wavelengths thatare different from those in FIG. 4 can be ascertained from theseresults.

[0076]FIG. 7 is a diagram representing, according to a simulation, thefunction of a Faraday rotator that selectively rotates only thepolarization planes of incident light of three wavelengths.

[0077] The multi-layer film structure for FIG. 7 may be represented as1L(1H 1L)⁶2.2M4L 2M(1L 1H)⁶1L. The significance of the symbols thatrepresent the multi-layer structure is likewise as with FIG. 2.

[0078] When this multi-layer film structure is illuminated with infraredlight 1000 to 2000 nm in wavelength, as shown in FIG. 7, only incidentlight approximately 1330 nm in wavelength, approximately 1530 nm inwavelength, and approximately 1760 nm in wavelength resonates within themagneto-optical part; and incident light in the vicinity thereof, in awavelength region of from roughly 1250 nm to 1850 nm, is blocked. Fromthese simulation results, it is evident that plural incident beams ofthree wavelengths can be localized within a magneto-optical part byadjusting the thickness and layout of the magneto-optical part, and ofdielectric layers that are interlaminated into the magneto-optical part,in the multi-layer film structure for FIG. 2. That a Faraday rotatormade up of the multi-layer film structure in FIG. 7 acts to selectivelyrotate only the polarization planes of incident light of threewavelengths can be ascertained from these results.

[0079] From the simulation results in FIGS. 2 through 7, it is evidentthat the wavelength of, and the number of wavelengths of, incident lightwhose polarization planes may be rotated utilizing the Faraday rotator30 are controllable by adjusting the thickness and layout of themagneto-optical part 30-1 (including such additional dielectric layersas have been interlaminated therein) and the dielectric multi-layerfilms 30-2.

[0080] Thus from the foregoing, according to Embodiment 1, by means of aresonant structure in which the dielectric multi-layer films 30-2 arearranged on either side of the magneto-optical part 30-1, the Faradayrotator 30 is capable of localizing light of not only a singlewavelength, but also a plurality of wavelengths, within themagneto-optical part 30-1.

[0081] Moreover, being that the magneto-optical part 30-1 and thedielectric multi-layer films 30-2 are jointly a thin-film structure,integrating them both is possible by means of thin-film laminationtechnology. This accordingly makes possible miniaturizing, andcurtailing the cost of, the magneto-optical part 30-1, the dielectricmulti-layer films 30-2, and the Faraday rotator 30 in which they bothare assembled, and furthermore simplifies the Faraday rotator 30manufacturing process.

[0082] Embodiment 2

[0083]FIGS. 8 and 9 are views schematically illustrating opticalisolators of Embodiment 2 according to the invention.

[0084] Optical isolator 60 a in FIG. 8 is constructed by arranging apolarizer 20 and an analyzer 40 on either side of the Faraday rotatordepicted in Embodiment 1, and further arranging magnetic parts 5 alongthe top and bottom.

[0085] As explained in setting out Embodiment 1, the Faraday rotator 30functions to selectively rotate only the polarization plane of incidentlight of a given wavelength(s).

[0086] This enables optical isolator 60 a incorporating the Faradayrotator 30 to selectively block only the return beams from the incidentlight of the given wavelength(s).

[0087] The polarizer 20 and the analyzer 40 can be constituted byirradiating a diamond-like carbon (DLC hereinafter) thin film along abias with either a particle beam or an energy beam. (Details of apolarizer (analyzer) constituted using the DLC thin film will bedescribed in Embodiment 3.)

[0088] According to the foregoing, in addition to the Faraday rotator 30having a thin-film structure, inasmuch as the polarizer 20 and theanalyzer 40 can also be rendered in a thin-film structure, integratingthem by means of thin-film lamination technology enables miniaturizingand reducing the cost of the optical isolator 60 a, and likewise allowsthe manufacturing process to be simplified.

[0089] Optical isolator 60 b in FIG. 9 utilizes as magnetic parts 50 agallium-nitride magnetic semiconductor thin film that exhibitsferromagnetism at room temperature. As shown in FIG. 9, the opticalisolator 60 b is structured by arranging the magnetic parts 50 on theouter sides of the polarizer 20 and the analyzer 40.

[0090] Because the gallium-nitride magnetic semiconductor thin film istransparent to light, it can be disposed in the path of the incidentlight beam.

[0091] This means that in addition to the polarizer 20, the Faradayrotator 30, and the analyzer 40, the magnetic parts 50 can be disposedin a straight line as shown in FIG. 9. Accordingly, integrating theseusing thin-film lamination technology enables miniaturizing andcurtailing the cost of optical isolator 60 b further, compared withoptical isolator 60 a, and likewise lets the manufacturing process besimplified that much more.

[0092] Embodiment 3

[0093]FIG. 10 is a view schematically illustrating a manufacturingprocess of a polarizer according to Embodiment 3 of the invention.

[0094] The polarizer is characterized in being formed by irradiating aDLC thin film 11 along a bias with either a particle beam or an energybeam. Although ion beams, electron beams, proton beams, α-rays andneutron beams are conceivable particle beams, and light rays, X-rays andγ-rays are conceivable energy beams, taking the example herein ofirradiating with ion beams, a method of lending a refractive indexdistribution to the DLC thin film will be explained with reference toFIG. 10.

[0095] As indicated in FIG. 10, at first a mask 12 that is atranscription of a refractive-index distribution pattern is adhered atopthe DLC thin film. From above the mask 12, oblique irradiation isperformed with a beam of helium or argon ions, for example. Therefractive index of the portions as at 11-1 receiving ion-beamirradiation through the transmitting areas of the mask 12 changes.Meanwhile, the refractive index of portions as at 11-2 cut off from theion-beam irradiation by the blocking areas of the mask 12 does notchange. Therefore, the refractive-index distribution of the DLC thinfilm may be controlled by changing the mask pattern. Exploiting thiseffect makes it possible to confer discrete polarization characteristicsin the DLC thin film 11.

[0096] Here, the fact that by ion irradiation of a DLC thin filmcontaining hydrogen, its refractive index can be altered within in arange extending from 2.0 to 2.5 has been reported in the journal Diamondand Related Materials, No. 7, 1998, pp. 432 to 434. It should also beunderstood that altering the refractive index through particle-beamirradiation such as ion irradiation, or through energy-beam irradiation,is not limited to hydrogen-containing DLC thin films, but is possiblewith, e.g., nitrogen-containing DLC thin films, and is possible with DLCthin films containing neither.

[0097] In this respect, the performance of a polarizer utilizing a DLCthin film was simulated with reference to the report that by ionirradiation of a DLC thin film containing hydrogen, its refractive indexcan be altered within in a range extending from 2.0 to 2.5. Thesimulation was carried out under a setting in which a DLC thin film—inwhich 25 laminae each, 50 laminae total, of a high refractive-indexlayer (refractive index 2.5) with a single lamina being 152.5 nm, and alow refractive-index layer (refractive index 2.0) with a single laminabeing 190.63 nm, were laminated in alternation—was illuminated with aninfrared beam, 1000 nm to 2000 nm in wavelength, at an incident angle of65 degrees. Graphically represented in FIG. 11 are the results of thissimulation.

[0098] From FIG. 11 it is evident that the polarization extinction ratioat 1300 nm reaches approximately −35 dB. That fabricating a polarizer(analyzer) is possible by carrying out particle-beam irradiation such asion irradiation, or energy-beam irradiation, at a bias on a DLC thinfilm may be ascertained from these results.

[0099] Next, DLC thin-film fabrication conditions will be discussed.

[0100] Existing techniques for film-formation of DLC thin filmsincorporating hydrogen include every sort of CVD(chemical-vapor-deposition) method employing heat or plasma, thesputtering method, the EB (electron-beam) deposition method, and thearc-ion-plating method (filtered-arc method). In practice, nevertheless,CVD methods, by which high-speed film formation is feasible, would seemto be the most suitable, given that they allow the introduction of alarge quantity of hydrogen into the film, and that a film thickness onthe order of 20 μm is required. In this respect, film formation by theparallel-plate plasma CVD method will be taken up.

[0101] As an example of the film-formation conditions with the parallelplate plasma CVD method: for substrate size, a 30-cm square; forfilm-formation-substrate temperature, 200 degrees centigrade, andpressure, 1.3×10¹ to 1.3×10⁻¹ Pa; for flow-volume of methane as theprecursor gas, 100 sccm; apply a high frequency of 13.56 MHz at a powerof approximately 100 W. Vacuum vessel: rotary pump and expansion pump,pressure-control with an orifice.

[0102] With either a particle beam or energy beam, irradiating along abias a DLC thin film fabricated under the fabrication conditions notedabove enables altering the refractive index of the DLC thin film.Controlling the refractive-index modification makes it possible toutilize the DLC thin film as a polarizer.

[0103] As in the foregoing, according to Embodiment 3, a polarizeradopting a thinly formed structure, yet laminated and integrated withother thin-film optical elements, may be fabricated by irradiating a DLCthin film along a bias, with either a particle beam or an energy beam.

[0104] Embodiment 4

[0105]FIG. 12 is a chart diagramming measurement results on the spectraltransmission characteristics of a DLC thin film that, using theparallel-plate plasma CVD method, was actually fabricated. The DLC thinfilm was formed to have a film thickness of 1.0 μm onto a glasssubstrate 1.5 mm thick. Here, the DLC thin film was fabricated byaltering the film-formation conditions under the parallel-plate plasmaCVD method explained in Embodiment 3, to enhance its hydrogenconcentration.

[0106] As indicated in FIG. 12, the DLC thin film fabricated in thisinstance has spectral transmission characteristics near 100% withrespect to light of from 500 nm to 2000 nm in wavelength, which includesthe wavelengths for optical communications. It should be understood thatthe spectral transmission characteristics in FIG. 12 are the “DLCthin-film internal transmittance,” from which the influences ofreflection at the obverse face of the DLC thin film, the reverse face-ofthe glass substrate, and the boundary surface between the DLC thin filmand the glass obverse face have been removed.

[0107]FIG. 13 is a chart diagramming the DLC thin film's opticalcharacteristics, calculated based on the measurement results in FIG. 12.

[0108] As indicated in FIG. 13, it will be understood that at forexample a 1500 nm wavelength hypothesized for optical communications,the DLC thin film fabricated in this instance has a refractive indexn=1.55, and an extinction coefficient k=4.48×10⁻⁵.

[0109] Meanwhile, pages 1758 to 1761 in the journal Diamond and RelatedMaterials, No. 9, 2000 contain a recent representative article onmeasuring the optical characteristics of DLC. This article carries theabsorption coefficient for DLC following helium ion irradiation with a1.0×10¹⁶ cm⁻² dose, as data pertaining to the DLC absorption coefficientat wavelength 1500 nm. The extinction coefficient k at wavelength 1500nm was calculated based on this DLC absorption coefficient, whereupon itturned out to be k=4×10⁻⁴.

[0110] Accordingly, that at the 1500 nm wavelength hypothesized foroptical communications, the DLC thin film fabricated in this instancehas a remarkably low extinction coefficient compared with conventionalDCL was verified. Furthermore, it can be read from FIG. 13 that even fora wavelength not only of 1500 nm, but also in the range of 1200 nm to1700 nm, the extinction coefficient for the DLC thin film fabricated inthis instance is 3×10⁻⁴ or less, which is lower than the 4×10⁻⁴ ofconventional DLC. Advantages such as that the lower the extinctioncoefficient, the less is the signal attenuation in, e.g., the opticalcommunications field will be appreciated.

[0111] Thus, the DLC thin film fabricated in this instance, which hassuperior characteristics not present to date, should have potentialapplied uses not only in optical communications, but also the polarizerdescribed in Embodiment 3 to begin with, and in various otherapplications.

[0112] The modes of embodying disclosed on this occasion should beconsidered exemplifications in ail respects, not limitations. The scopeof the present invention is not the explanation set forth above, but isindicated by the scope of the claims; and the inclusion of meaningsequivalent to the scope of the claims, and all changes within the scope,is intended.

[0113] As thus in the foregoing, under this invention, miniaturizing andmoreover holding down the costs of Faraday rotators, polarizers,analyzers, magnetic parts, and optical isolators having these as theirconstituent elements, is made possible. Likewise, manufacturing Faradayrotators and optical isolators that can handle plural wavelengths ismade possible. Furthermore, a new material useful in miniaturizing andin lowering the cost and enhancing the performance of polarizers tobegin with, and of various sorts of optical devices, can be provided.

What is claimed is:
 1. A Faraday rotator having wavelength selectivity,for selectively rotating only the polarization plane of incident lightof given wavelengths, the Faraday rotator comprising: a magneto-opticalpart for rotating the polarization plane of incident light traveling inthe direction of said magneto-optical part's magnetic field; and adielectric multi-layer film in which a low refractive-index layer and ahigh refractive-index layer are laminated in alternation, for localizingwithin said magneto-optical part incident light of at least onewavelength.
 2. The Faraday rotator set forth in claim 1, wherein saiddielectric multi-layer film localizes within said magneto-optical partincident light of plural wavelengths.
 3. The Faraday rotator set forthin claim 1, wherein said magneto-optical part is constituted from agadolinium iron garnet thin film.
 4. The Faraday rotator set forth inclaim 1, wherein said dielectric multi-layer film is composed bylaminating in alternation silicon oxide as a low refractive-index layer,and titanium oxide as a high refractive index layer.
 5. The Faradayrotator set forth in claim 1, wherein said magneto-optical part and saiddielectric multi-layer film are formed integrally by a vapor-phaseprocess.
 6. An optical isolator having wavelength selectivity, forselectively blocking return beams from incident light of givenwavelengths only, the optical isolator comprising: a magneto-opticalpart for rotating the polarization plane of incident light traveling inthe direction of said magneto-optical part's magnetic field; a magneticpart for applying a magnetic field to said magneto-optical part; adielectric multi-layer film in which a low refractive-index layer and ahigh refractive-index layer are laminated in alternation, for localizingwithin said magneto-optical part incident light of at least onewavelength; a polarizer for picking out polarized components fromincident beams; and an analyzer utilized in combination with saidpolarizer.
 7. The optical isolator set forth in claim 6, wherein saiddielectric multi-layer film localizes within said magneto-optical partincident light of plural wavelengths.
 8. The optical isolator set forthin claim 6, wherein said magneto-optical part is constituted from agadolinium iron garnet thin film.
 9. The optical isolator set forth inclaim 6, wherein said magnetic part is constituted from agallium-nitride magnetic semiconductor thin film that exhibitsferromagnetism at room temperature and is transparent to light.
 10. Theoptical isolator set forth in claim 6, wherein said dielectricmulti-layer film is composed by laminating in alternation silicon oxideas a low refractive-index layer, and titanium oxide as a high refractiveindex layer.
 11. The optical isolator set forth in claim 6, wherein saidpolarizer and said analyzer are lent a structure having distributedrefractive indices, by irradiating with either a particle beam or anenergy beam a diamond-like carbon thin film along a bias with respect tothe film's thickness direction.
 12. The optical isolator set forth inclaim 11, wherein said particle beam is an ion beam, an electron beam, aproton beam, α-rays, or a neutron beam; and said energy beam is lightrays, X-rays or γ-rays.
 13. The optical isolator set forth in claim 6,wherein said magneto-optical part, said magnetic part, said dielectricmulti-layer film, said polarizer, and said analyzer are formedintegrally by a vapor-phase process.
 14. A polarizer lent acharacteristic structure having distributed refractive indices, byirradiating with either a particle beam or an energy beam a diamond-likecarbon thin film along a bias with respect to the film's thicknessdirection.
 15. The polarizer set forth in claim 14, wherein saidparticle beam is an ion beam, an electron beam, a proton beam, α-rays,or a neutron beam; and said energy beam is light rays, X-rays or γ-rays.16. A diamond-like carbon thin film characterized in being transparentin the light region, and in having an extinction coefficient that is3×10⁻⁴ or less at optical-communications wavelengths of from 1200 nm to1700 nm.
 17. An optics component, characterized by utilizing thediamond-like carbon thin film set forth in claim
 16. 18. The opticalisolator set forth in claim 11, wherein said diamond-like carbon thinfilm is transparent in the light region, and has an extinctioncoefficient that is 3×10⁻⁴ or less at optical-communications wavelengthsof from 1200 nm to 1700 nm.
 19. The optical isolator set forth in claim12, wherein said diamond-like carbon thin film is transparent in thelight region, and has an extinction coefficient that is 3×10⁻⁴ or lessat optical-communications wavelengths of from 1200 nm to 1700 nm. 20.The polarizer set forth in claim 14, wherein said diamond-like carbonthin film is transparent in the light region, and has an extinctioncoefficient that is 3×10⁻⁴ or less at optical-communications wavelengthsof from 1200 nm to 1700 nm.
 21. The polarizer set forth in claim 15,wherein said diamond-like carbon thin film is transparent in the lightregion, and has an extinction coefficient that is 3×10⁻⁴ or less atoptical-communications wavelengths of from 1200 nm to 1700 nm.